This invention relates to a color image forming apparatus, and more particularly to a color image forming apparatus capable of reproducing a color image with high reproducibility irrespective of color distribution of the color image.
There has been known an image forming apparatus in which an original color image on an image input medium such as a color photographic film, a television, a copy sheet or the like is read out by an image reader to obtain an original image data, the original image data is separated into three color components (red, green and blue image data) for every picture element. The original image data is transformed or color-proofed into a control signal data. The control signal data controls a recording means of the image forming apparatus to record, on an image output medium, a color image which has the same colors as the original image formed on the image input medium, as a result of which the original image formed on the image input medium is reproduced on the image output medium. The image output medium is such as a copy sheet, a television, a coated or non-coated sheet or the like.
In this description, each of the image input media and the image output media is defined as a medium for forming or producing a visible color image thereon. The medium is referred to as "image input medium" when the medium is used as an original from which an original image is read by the image reading unit, while the medium is referred to as "image output medium" when the medium is used as a recording medium on which the original image is reproduced.
Color-reproducible ranges of the image input and output media are determined in accordance with kinds of image forming manners or image forming systems utilized in the image input and output media. Kinds of the image input and output media such as the color photographic film, copy sheet, television, printed sheet or the like are determined in accordance with the kinds of the image forming systems utilized in the image input and output media. Therefore, the color-reproducible ranges of the image input and output media are determined in accordance with kinds of the image input and output media. In the above-described image forming apparatus, inconsistency in color (color error or color difference) between the original image and the image reproduced on the image output medium frequently occurs when a color-reproducible range (gamut) of the image input medium is inconsistent with that of the image output medium. A color located within the color-reproducible range (gamut) of the image input medium is reproduced on the image output medium with insufficient color-reproducibility, in the case where the color of the image input medium is located outside of the color-reproducible range (gamut) of the image output medium.
Various techniques (color-correcting techniques) for correcting an original image data have been utilized to correct such a color-inconsistency (decrease such a color error) and improve the color-reproducibility of the original image.
As one of the color-correcting techniques is known a color compression technique (a data-compression transform technique). In the data-compression transform technique, a color represented by a color information signal data is compressed in an achromatic (neutral) color direction under an invariable compression rule to obtain a color-compressed color which is located within the color-reproducible range of the image output medium and therefore can be recorded or reproduced on the image output medium. Thus obtained data of color-compressed color is color-proofed to produce the control signal for driving the recording means to record or reproduce the color-compressed color on the image output medium. Thereafter, a color recording operation is carried out using cyan, magenta and yellow inks or toners by the recording means in accordance with the obtained control signal data.
In this description, the achromatic color is defined as a color having a lightness, but no hue and chromaticness (saturation). White, gray and black colors belong to the achromatic color, and are decreased in lightness in this order. In the following description, a color axis on which achromatic colors having various lightnesses (white, gray and black) are located is referred to as "achromatic color axis". The achromatic color on the achromatic color axis is shifted up to the white color as a lightness thereof is increased, and down to the black color as the lightness is decreased. An achromatic color direction is defined as a direction in which any color is directed to an achromatic color on the achromatic axis in a color space. As described below, the color-compression process is a data-correcting process for shifting (color-compressing) a color within the color-reproducible range (gamut) of the image input medium to a color within the color-reproducible range (gamut) of an image output medium in the color space.
As described above, in order to record or reproduce an original image whose color is within a color-reproducible range (gamut) of the image input medium but is out of a color-reproducible range (gamut) of the image output medium, in the color-compression process, a color image data of the original image is corrected such that the color of the original image is shifted to a suitable color within the color-reproducible range (gamut) of the image output medium in the chromaticity diagram (color space). In other words, the original image data is corrected such that the color of the original image is shifted in a direction toward a point on the achromatic color axis to produce a color located within a color-reproducibility of the image output medium. This color-correcting process is hereinafter referred to as "a color-compression process in an achromatic color direction".
In the conventional image forming apparatus adopting the above color-compression process, a color-compression parameter with which the original image data is color-compressed is beforehand determined as an invariable value for attaining a predetermined invariable compression rule, and is beforehand stored in a memory means of the apparatus.
In general, each of original color images formed on the image input media has an inherent color distribution. That is, the colors of each original color image falling within the image input medium gamut has an inherent distribution with respect to the image output medium gamut. More specifically, each of the original color images has an inherent ratio or degree of colors located within the image output medium gamut with respect to other colors located outside of the image output medium gamut. In other words, each of the original color images has an inherent ratio of colors which can be recorded by the recording means and other colors which may not be recorded by the recording means. Thus, the ratios of colors recordable by the recording means with respect to colors unrecordable by the recording means are different from one another, in accordance with the original color images.
The conventional compression technique as described above, however, subjects all of the original images to the predetermined invariable compression process utilizing the invariable color-compression parameter irrespective of the difference of the color distribution of the original color images, and thus causes some original images to be reproduced with insufficient contrast due to an excessive color-compression or to be reproduced with saturation colors due to insufficient color compression, with the result that a sufficient color reproducibility is not obtained.